Rongtao Su

1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality.

In this manuscript, we demonstrate high power, all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality by simultaneously suppressing detrimental stimulated Brillouin scattering (SBS) and mode instability (MI) effects. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using three-stage cascaded phase modulation systems. Output powers of 477 W, 1040 W, and 1890 W are achieved with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The MI threshold is increased from ~738 W to 1890 W by coiling the active fiber in the main amplifier. Both the polarization extinction ratio (PER) and beam quality (M2 factor) are maintained well during the power scaling process. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and near-diffraction-limited beam quality at 2 kW power-level.

Single-frequency 332 W, linearly polarized Yb-doped all-fiber amplifier with near diffraction-limited beam quality

A 332 W all-fiber amplifier chain with single frequency and single polarization (average extinction ratio >21 dB) is presented based on master oscillator power amplification configuration. A highly Yb-doped concentration fiber of 2.8 m with a core diameter of 30 μm is utilized as the gain fiber in the final fiber amplifier. The slope efficiency of the main amplifier is measured to be 83.7% and the beam quality (M² factor) is measured to be 1.4. By measuring the polarization extinction ratio (PER) of the amplifier in a different power, we conclude that the PER degrades little as the output power enhances. The output power of backward light reveals that stimulated Brillouin scattering effect is not induced in the configuration. To the best of our knowledge, this is the highest power of all-fiber single frequency and single polarization amplifier with near-diffraction-limited beam quality.

Active coherent beam combining of a five-element, 800 W nanosecond fiber amplifier array

Coherent beam combining of an 800 W nanosecond single-frequency fiber amplifier array has been demonstrated. Five high-power all-fiberized Yb-fiber amplifiers were tiled into a crisscross array, and the laser beams were actively phase-locked by using the stochastic parallel gradient descent algorithm. The contrast of the far-field intensity pattern of the combined beam was more than 91.6%. The combined pulsed laser has a pulse width of ~3.5 ns and a peak power of 21.5 kW.

350-W Coherent Beam Combining of Fiber Amplifiers With Tilt-Tip and Phase-Locking Control

We demonstrate the coherent beam combining (CBC) of a 350-W single-frequency fiber amplifier array with simultaneous tilt-tip control and phase-locking control. An adaptive fiber-optics collimator is employed to perform the tilt-tip control, and the phase-locking control is realized by the phase modulator. The fiber array in the CBC system consisted of two high-power fiber amplifiers, each with output power of about 175 W. Stochastic parallel gradient descent algorithm is employed in both tilt-tip and phase-locking control. The tilt-tip control makes two separate laser beams overlap with each other in the far field, while the phase-locking control compensates the piston phase differences between the fiber lasers. Normalized power in bucket increases from 0.27 in open loop to 0.99 in closed loop. The fringe contrast is improved from 0% to more than 97% correspondingly. The tilt-tip control bandwidth is higher than 5.4 Hz when the tilt-tip mean square error is 10 μrad.

150 W high-average-power, single-frequency nanosecond fiber laser in strictly all-fiber format.

We demonstrate an all-fiber, single-frequency nanosecond laser with both high peak power and average power based on a master oscillator power amplifier (MOPA) configuration. The MOPA produced a single-frequency pulsed laser with pulse duration of ∼8  ns. The average and peak power were as much as 139.3 W and 1.07 kW, respectively, when the repetition rate was 10 MHz, and 153.1 and 668 W, respectively, when the repetition rate was 20 MHz. Higher output power can be obtained by increasing pump power of the main amplifier.

Generation of azimuthally and radially polarized beams by coherent polarization beam combination.

A new architecture for generating pure azimuthally and radially polarized beams is presented. It involves coherent polarization beam combination of two orthogonally polarized LP(11) fiber modes. Experimental results reveal that high purely polarized (polarization purity of 95% or better) azimuthal and radial beams can be generated.

Coherent polarization beam combination of four mode-locked fiber MOPAs in picosecond regime

In this manuscript, we report on coherent polarization beam combination (CPBC) of a four-channel pulsed amplifier array in the picosecond regime by using single frequency dithering technique. By employing a photo-detector with low-pass bandwidth (8.5 MHz at 10 dB gain) to filter the intensity fluctuation and obtain phase errors for feedback, a combined laser pulse with~480 ps pulse width at~60 MHz repetition rate is obtained with an average power of 88 W. By adjusting the optical path differences (OPDs) and controlling the pump power to ensure the synchronizations and alleviate the influence of nonlinear phases among each channel, more than 90% combining efficiency is achieved with excellent beam quality (M(2)~1.1). Finally, the efficiency loss of the system along with the power scaling process is discussed.

Active coherent beam combination of two high-power single-frequency nanosecond fiber amplifiers

We report on active coherent beam combining of two single-frequency nanosecond fiber amplifiers by using stochastic parallel gradient descent algorithm. Each fiber amplifier produces an average power of more than 100 W by using three-stage cascaded amplification. The two laser beams are actively phase-locked with a total average power of 215.8 W, producing ~8 ns pulses at 10 MHz repetition rate. The contrast of the far-field intensity pattern of the coherently combined beam is more than 83%.

Generation of a 481-W Single Frequency and Linearly Polarized Beam by Coherent Polarization Locking

We demonstrate active coherent polarization locking of two high power polarization-maintained fiber amplifiers using single frequency dithering technique. When the active phasing system is in the closed loop, a 481-W single frequency and polarization-maintained laser beam with near-diffraction-limited beam quality is generated in the circumstance of imbalanced power ratio of the two amplifiers. The beam quality ( M2 factor) of the coherently polarization locked beam is measured to be and the polarization extinction ratio of the phasing beam can be as high as 10 dB (91%).

High-power coherent beam polarization combination of fiber lasers: progress and prospect [Invited]

Coherent polarization beam combining (CPBC) of fiber lasers has the potential to scale the output total power while simultaneously maintaining good beam quality. In this paper, we will present the very recent technical advance in a single-channel coherently combinable linearly polarized narrow-linewidth fiber amplifier and high-power CPBC system. The noise property of a recently developed near 2 kW fiber amplifier and its feasibility in a CPBC system, CPBC of four 500-W-level fiber amplifiers and two kilowatt fiber amplifiers are demonstrated for the first time, which is also the first result for a 2 kW CPBC system. We have also performed numerical analysis on the performance scaling of CPBC, and deduced handy design guidelines for CPBC of a 64-element high-power system.